Simply put, ventilation is a system that provides outdoor air to a building. There are two
types of ventilation: mechanical ventilation actively moves outdoor air into a building, and natural ventilation relies on the passive flow of air from outdoor to indoor spaces. Ventilation moves outdoor
air into a building to replace any pollutants that might have accumulated in indoor air and is thus an integral aspect of indoor air
quality (IAQ). In mechanically ventilated buildings, the supply air coming out of an air diffuser or other supply vent is
typically a mixture of outdoor air and air that has recirculated through the building. Naturally ventilated buildings are designed and built in a way that relies on natural pressure differences to move external air to an indoor space. Most homes are said to be naturally ventilated, in that outdoor air comes indoors through open doors and windows and because most homes are "leaky." Heating and/or cooling systems in homes typically recirculate the same air after treating (heating or cooling) and are thus not part of the official ventilation system since they are not bringing in outdoor air.
Ventilation can impact indoor air quality and health
Ventilation is an important control strategy for maintaining good IAQ and improving poor IAQ. Properly designed ventilation can dilute
particles and gases in indoor air and prevent contaminants from accumulating to
levels that may cause health or comfort problems. It can also remove excessive
moisture and prevent microbial growth.
Ventilation should be used in conjunction with source control, by preventing
contaminants from entering the air, and, in some cases, air cleaning (see Summary of Available Information on Residential Air Cleaners from U.S. EPA ). In general, it is more efficient to eliminate
or reduce sources of indoor pollutants or use local exhaust ventilation or air
cleaning devices to remove them near their source than to control them by
general ventilation. For instance, exhaust fans over kitchen stoves and in bathrooms are effective ways to remove combustion products and excess moisture, respectively, from those point sources.
Ventilation rates that are too low can cause pollutants
and moisture to build up to levels that may lead to unpleasant odors,
discomfort, or even adverse health effects. Lower ventilation rates have been
associated with higher symptom rates and reduced performance in school or work
settings. Therefore, higher ventilation rates are generally desirable from the
IAQ point of view. However, there are two reasons not to provide exceedingly
high ventilation levels:
1) Increasing the ventilation rate may increase
energy consumption if the outdoor or recirculated air must be "conditioned",
that is, cooled or heated and perhaps dehumidified or humidified.
2) If a
contaminant is present at higher levels in the outdoor air that is brought into
a building (for example, ozone or pollutants exhausted from vehicles), then
increasing the ventilation rate may bring in pollutants more harmful than those
that are being removed, unless the air coming in is treated to remove the
contaminants from outdoors.
In practice, building ventilation rates need to balance energy consumption
with the known or expected health and comfort benefits. More information
regarding the effect of ventilation rate on IAQ, health, and productivity can be
found at the website of Lawrence Berkeley National Laboratory's Indoor Environment Department.
Standards and codes establish minimum ventilation rates to provide
comfortable indoor environments and protect human health. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends minimum ventilation rates for different types of buildings and spaces (known as
ASHRAE Standards 62.1 and 62.2), which are often incorporated
into national, state, and local codes. Typically standards and codes specify the
minimum ventilation rate per person or per unit of floor area. The minimum rate varies with the type of building or usage
of the space; for example, a higher air change rate is recommended for health
club weight rooms (26 cfm/person based on default occupant density) than for
office spaces (17 cfm/person) because the higher activity levels in the weight
room will produce more occupant-related odors. The Code of Regulations
identifies the minimum ventilation rates required for California buildings; see California's Energy Efficiency Standards for Residential and Nonresidential Buildings.
California workers are fortunate in that not only must workplace mechanical
ventilation systems be designed and built to meet code requirements, the systems must also: (i) be operated to provide at least the quantity of outdoor air
required by code, (ii) be operated continuously during working hours (with
exceptions), (iii) be inspected at least annually and problems corrected within
a reasonable time, (iv) be maintained, and (v) records of all system inspections
and maintenance be kept in writing and available within 48 hours of request (see
the Cal/OSHA Minimum Ventilation Standard).